This invention relates generally to injection molding and more particularly to a manifold having a heating element and a branching portion of a melt passage which extend in transverse planes not spaced far apart and a method of making it.
It is well known to have an injection molding manifold for a multi-cavity molding system in which the melt passage branches from a single inlet to a plurality of outlets. As seen in U.S. Pat. No. 4,648,546 to Gellert which issued Mar. 10, 1987, this is made by machining two matching grooves in the abutting surfaces of two steel plates and then brazing the plates together so the matching grooves form the branching portion of the melt passage. U.S. Pat. No. 4,648,546 also shows the manifold having an integral electrically insulated electrical heating element. However, the heating element is embedded in a channel in the front face of the manifold and thus is spaced a considerable distance from the branching portion of the melt passage. This has the disadvantage that the temperature gradient in different parts of the manifold due to heat loss to the nozzles and the surrounding mold results in the temperature of the branching portion of the melt passage not being sufficiently uniform and constant. This is particularly a problem for molding materials having a narrow temperature window. Furthermore, a very rapid heat up can result in slight bowing of the manifold.